The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device. A reductant is typically sprayed or injected into hot exhaust gases upstream of the SCR device. The reductant may be an aqueous urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is absorbed by the SCR device. The ammonia then reduces the NOx to nitrogen in the presence of the SCR catalyst. The distribution of the reductant upstream of the SCR device in a cross-stream direction has a significant impact on the overall NOx reduction efficiency by the SCR catalyst. Specifically, the uniformity of the reductant distribution affects the SCR performance, where a more uniform distribution of reductant in the exhaust gas typically results in the SCR catalyst being able to reduce NOx to nitrogen more effectively. Thus, if the reductant distribution in the exhaust gas is non-uniform, then the SCR catalyst performance may be limited. A non-uniform distribution of reductant may also result in formation of cyanuric acid, melamine, biuret, and other solid reaction products along the inner walls of the exhaust gas piping, which is generally unwanted.
One approach for improving the reductant distribution is to provide multiple mixers or turbulators in the exhaust gas conduit, as well as to increase the length of the exhaust gas conduit. However, this approach adds extra weight, cost, and thermal mass to the exhaust gas system. Accordingly, there exists a need for a cost-effective approach for providing a relatively uniform distribution of reductant to an SCR device.